(1) Field of the Invention
The present invention relates to speakers and electronic devices including the same, and more particularly to a slim and thin speaker and an electronic device including the same.
(2) Description of the Related Art
In recent years, with the popularity of high-definition and wide-screen television and the like, wide and large television screens have been widely used. On the other hand, there is a demand for narrow and thin television sets to satisfy the housing situations in Japan.
Speaker drivers (hereinafter, referred to simply as “speakers”) used in a television set are generally provided at both sides of a TV display screen such as cathode-ray tube (CRT) display. The speakers therefore results in increase of a width of the television set. In order to address the above drawback, TV speakers have been slim in shape, for example, having a horn or elliptic shape (see Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-32659). Furthermore, with the increase of width of TV screens such as CRT displays, speakers are demanded to be further narrower and to have higher sound quality according to higher image quality of the screen. Moreover, thin television sets having a plasma display or a liquid crystal display are increased. Therefore, thinner speakers are further demanded.
The following describes the conventional slim speaker disclosed in Patent Reference 1 with reference to figures. FIGS. 24A to 24C are views of the conventional slim speaker. FIG. 24A is a plane view of the conventional slim speaker. FIG. 24B is a cross-sectional view of the conventional slim speaker taken along line C-C′ (in other words, along a longer side of the speaker, hereinafter, referred to also as “along the longer-side direction”) of FIG. 24A. FIG. 24C is a cross-sectional view of the conventional slim speaker taken along line O-O′ (in other words, along a shorter side of the speaker, hereinafter, referred to also as “along the shorter-side direction”) of FIG. 24A.
The slim speaker shown in FIGS. 24A to 24C includes a magnet 801, a plate 802, a yoke 803, a frame 804, a voice coil bobbin 805, a voice coil 806, a damper 807, a diaphragm 809, a dust cap 810, and an edge 811. In this slim speaker, there is a magnetic gap 808 between: the magnet 801 and the plate 802; and the yoke 803.
The voice coil 806 is a conductor wire made of copper, aluminium, or the like. The voice coil 806 is fixed to the voice coil bobbin 805 that is cylindrical in shape. In other words, the voice coil bobbin 805 suspends the voice coil 806 in the magnetic gap 808.
The voice coil bobbin 805 is connected to the frame 804 via the damper 807. Furthermore, the voice coil bobbin 805 is adherently fixed to the side of the diaphragm 809 which is opposite to the side fixed with the voice coil 806.
The diaphragm 809 is in a shape of an ellipse or a substantially ellipse. A dust cap 810 is adherent to the center of the diaphragm 809. A cross-sectional surface of the dust cap 810 is substantially semicircular in shape.
The edge 811 is circular in shape. A cross section of the edge 811 is a semicircle. The inner periphery of the edge 811 is fixed to the outer periphery of the diaphragm 809. The outer periphery of the edge 811 is adherent to the frame 804.
Next, the description is given for driving of the speaker having the above-described parts as shown in FIG. 24A to 24C. First, electric current (current) flows into the voice coil 806. The current flowing in the voice coil 806 and a magnetic field around the voice coil 806 allows the voice coil bobbin 805 to execute pistonic motion. Thereby, the diaphragm 809 moves back and forth (vibrates) by the pistonic motion. As a result, the diaphragm 809 produces sound waves.
FIG. 25 is a graph plotting frequency characteristics regarding a reproduction sound pressure level of the conventional speaker disclosed in Patent Reference 1. In FIG. 25, a vertical axis represents a reproduction sound pressure level and a horizontal axis represents a drive frequency, in the situation where electric power of 1 W is applied to the speaker. The reproduction sound pressure level is measured by a microphone far by 1 m from a front of the speaker on the center axis of the speaker.
However, the conventional speaker has the following problem. More specifically, the speaker shown in FIGS. 24A and 24C employs a driving method of driving the center portion of the slim diaphragm 809. Therefore, breakeup resonance is likely to occur along a longer-side direction of the diaphragm 809. As a result, the frequency characteristics regarding reproduction sound pressure level have peaks and dips in middle and high ranges. In other words, the above-described conventional speaker causes breakeup resonance along the longer-side direction of the speaker and thereby deteriorates sound quality. As seen in FIG. 25, the problem is apparent from noticeable dips (peak/dip) closer to driving frequency of 2 kHz (A in FIG. 25), driving frequency of 3 kHz (B in FIG. 25), and driving frequency of 5 kHz (C in FIG. 25).